Although Norwalk virus and Norwalk-like agents have been shown to be the most common cause of epidemic nonbacterial gastroenteritis, at present almost nothing is known about the molecular epidemiology of these 27 nm viruses. Our goal is to determine what molecular characteristics are important in the epidemiology, immunogenicity, and pathogenesis of these agents. Thus far, we have cloned and sequenced part of the polymerase region of several 27 nm viruses from the Middle East and Canada to compare with that of the Norwalk virus which has just recently been sequenced. Also, we have now amplified the capsid region of the Norwalk virus by PCR and plan to do the same for other 27 nm viruses in order to generate clones for use in future protein expression studies.